Electron beam apparatus with beam-stabilization system

ABSTRACT

A high-voltage electron microscope is provided with a double focussing bending magnet for stabilizing the beam velocity. The beam path between an entrance diaphragm in front of the magnet and an image produced by the bending magnet of this diaphragm on the exit side of the magnet are used as a reference for the signal to be detected on the output side of the bending magnet. Near this image, a detector device is provided which intercepts a fraction of from 1 to 10 percent of the beam from which it derives a control signal for the high voltage.

United States Patent Rus et a1.

[ Oct. 23, 1973 1 ELECTRON BEAM APPARATUS WITH BEAM-STABILIZATION SYSTEM [75 lnventorsi Pieter Jan Rus; Jan Bart Le Poole;

' Johannes Kramer, all of Delft,

Netherlands [73] Assignee: U.S. Philips Corporation, New

York

[22] Filed: Aug. 25, 1971 [21] Appl. No.: 174,657

30 Foreign Application Priority Data Aug. 28, 1970 Netherlands 7012758 [52] U.S. Cl. 250/311, 250/397 [51] H0lj 37/26 [58] Field of Search 250/495 R, 49.5 E, 250/495 AE, 41.9 D, 41.9 ME, 311, 397; 324/71 EB, 71 CP [56] References Cited UNITED STATES PATENTS 3,012,139 12/1961 Hanson et a1 250/419 D Field 250/419 D 3,207,982 9/1965 Rose 250/419 D X 3,370,171 2/1968 Ohta 250/495 R X 2,777,958 1/1957 Le Poole 250/419 ME Primary Examiner-Walter Stolwein Att0rneyFrank R. Trifari [57] ABSTRACT A high-voltage electron microscope is provided with a double focussing bending magnet for stabilizing the beam velocity. The beam path between an entrance diaphragm in front of the magnet and an image produced by the bending magnet of this diaphragm on the exit side of the magnet are used as a reference for the signal to be detected on the output side of the bending magnet. Near this image, a detector device is provided which intercepts a fraction of from 1 to 10 percent of the beam from which it derives a control signal for the high voltage.

21 Claims, 4 Drawing Figures PAIENTEMcrzsma H 8.767.927 SHEET 1 BF 3 PIETER JAN RUS, POOIE JOHANNES KRAMER W IQ\ AGENT PAIENIEBucrzsms 3.767.927 sum 2 UF 3 PIETER JAN RUS, PooLm JOHANNES KRAMER AGENT ELECTRON BEAM APPARATUS WITH BEAM-STABILIZATION SYSTEM The invention relates to an electron beam apparatus which comprises a high-voltage generator, an electron source for generating an electron beam, a stabilisation system and an electron-optical imaging system.

In an electron beam apparatus of this kind the quality of an electron-optical image formation to be realized is co-determined to a large extent by the degree of cohstancy of the velocity at which the electron beam enters the imaging system. In known apparatuses, for example in high-voltage microscopes, the velocity of the electron beam is kept constant by utilizing a cascadegenerator as a'high-voltage source for the electron beam. Cascade generators for high voltages up to, for example, at least'l MV, require a large space for mounting and, together with the display system, this results in comparatively large dimensions for known electron beam apparatuses in electron microscopes.

The height of an electron beam apparatus of this kind requires special provisions with respect to the height of the space in which the apparatus is to be'installed, which involves additional cost.

The invention has for its object to provide, while maintaining a stability of the velocity of the electron beam which is sufficient for proper image formation, an electron beam apparatus of which in particular the height is considerably smaller than that of known apparatuses. The invention is based on the discovery that the requirements imposed can be satisfied by combining bending of the electron beam coupled with the use of a beam stabilisation system'for which the bending in the electron beam acts as a reference for the stabilisation.

In accordance with the invention, an electron beam apparatuses of the kind set forth is characterized in that the beam-stabilisation system comprises a doublefocussing magnetic beam bending device having a mag netic field at right angles to the electron beam for bending the electron beam which is accelerated by means of the accelerator tube.

path to be described by the electron beam between an I entrance diaphragm for the bending device and an image of this entrance diaphragm formed by the bending device is used in a preferred embodiment according to the invention as the reference for stabilizing the electron beam. After the electron beam has passed through the bending device a fraction of it is split off in dependence upon the position of the beam, and a signal derived therefrom by means of a detector can be used for adjusting the output voltage of the high-voltage generator.

Preferably there is disposed for this purpose near the exit of the bending device approximately at the location of the image of the entrance diaphragm, a collector element which intercepts a portion of the electron beam and, for example, scatters this portion, at least part of the scattered electrons being collected by the detector. The main portion of the electron beam then always remains available in the imaging system for the purpose of image-forming. If the output voltage of the high-voltage generator is sufficiently stable, a bending device according to the invention can be advantageously be used for position stabilisation of the electron beam.

FIG. 1 is a diagrammatic sectional view of a preferred embodiment of an electron beam apparatus according to the invention,

FIG. 2 shows schematically a preferred embodiment of a double-focussing bending magnet,

FIG. 3 is a cross-sectional view of a preferred embodiment of a double-focussing bending magnet, and

FIG. 4 is a block diagram of a preferred embodiment of a stabilizing system which forms part of an electronbeam apparatus according to the invention.

An electron-beam apparatus as shown in FIG. 1 has a Van de Graaff generator 1 which comprises a drive motor 3 provided with a magnetic screen 2, a conveyor belt 4 for transporting an electrical charge and a preferably cylindrical high-voltage electrode 5 as an output capacitor of the generator. An output voltage control device 6, an electron gun 7 for generating an electron beam 8, and an injection lens 9 are situated within the high-voltage electrode 5. Viewed from the electron gun 7, the injection lens 9 is followed by an accelerator tube 10 which comprises a series of, for example, 25 ring electrodes 11 which are at different porentials. The high-voltage generator 1, the electron gun 7, the injection lens 9, and the accelerator lens 10 are enclosed in a pressure tank 13 which may be provided with quickacting couplings 12. During operation a pressure of, for example, approximately 13 atmospheres prevails in the pressure tank. Within the pressure tank, electronoptical deflectors 14, for example electromagnetic deflector coils, may be provided for adjusting the electron beam 8 along an optical axis 15. Situated between the accelerator tube 10 and a subsequent electron-optical double-focussing bending device, in this case in the form of a 180 double-focussing bending magnet 16, is situated a diaphragm 17 as an entrance diaphragm for the bending device. By means of the bending device 14 the electron beam is passed through the diaphragm 17 in an optimum manner and is directed along the optical axis 15 of the system.

The diameter of the centre portion of the electron beam, which is virtually homogeneous as regards current density, is preferably approximately 5 pm larger than the largest diameter of the entrance diaphragm. A slight lateral-displacement of the electron beam with respect to the diaphragm then has no effect on the beam current transmitted by the diaphragm. A rotation-symmetrical diaphragm having a diameter of approximately um may be used as the entrance diamanner along the optical axis 15. In the case of deviations from the optical axis in a direction situated in a plane containing the bend of the beam, i.e., the plane of the drawings, the intercepted fraction varies about the value it has when the beam travels along the axis and a signal is obtained which is a function of the beam position at this point. The collector element 18 may be constructed so that the intercepted electrons are trapped and are carried off directly as a signal. In that case, the collector element must be electrically insulated and be provided with a signal output. The collector element may alternatively consist of a foil which scatters part of the incident electrons. In that case at least part of the scattered electrons are collected by a detector 19 disposed in the vicinity of the collector element. The detector is preferably provided with a semiconductor diode which is connected in the reverse direction and the conduction of which is affected by incident electrons (bombardment conduction). The output voltage control unit 6 is operated by a signal from the detector via electronic circuits. Owing to the provision of the bending magnets variations in the velocity of the electron beam result, in displacements of the beam (indicated by an arrow p in FIG. 2), and this displacement results in the detector signal by means of which velocity variations can be corrected. The control signal for the high voltage control can be rendered independent of the beam diameter and of the beam current by comparing the detector signal with permanently adjustable reference signals by electronic means. This may also be achieved by constructing the collector element of two oppositely arranged sharp edges which both bound the beam periphery in the direction of displacement. The detector for scattered electrons may be arranged so that the angle between the axis of the beam at the area of the collector element and a line joining the collector element and the detector is adjustable between approximately 20 and 80 C. The material composition of the semiconductor diode may be chosen such that by applying a desired reverse voltage a barrier layer is produced of sufficient thickness to impart an amplification of at least to the electrons which are collected by the diode and have an energy of approximately 10 eV. The portion of the electron beam which is not trapped or scattered by the collector element 18, i.e., approximately 90 to 99 percent, enters an imageproducing device 20. This device may be formed by any image producing device and comprises, for example, a first condenser 21, a condenser diaphragm 22, a second condenser 23, an objective lens 24, a specimen table 25, a selection diaphragm 26, a diffraction lens 27, a projector lens 28 and a shutter 29. A wall portion 30 of an envelope 31 of the image producing device accommodates a fluorescent screen 32, on which an electron image can be observed through a window 33 in the wall 31. The overall height of the vertically arranged electron-beam apparatus as shown in FIG. 1 is approximately 3 meters and is, consequently, approximately 3 meters lower than known apparatuses of this kind. It is not necessary to use a 180 bending magnet. By using a smaller bending angle the image screen can be disposed at a desired level. It is alternatively possible to use a reversing lens, for example as described in the published Netherlands Patent Specification No. 6,7l6,628, in the image producing device. It may then be advantageous to arrange the complete apparatus upside down relative to the situation shown in FIG. 1.

The position of the entrance diaphragm 17 is chosen to be such that the position of the collector element, which is determined by the image of the bending magnet, is advantageous for the remainder of the construction. The entrance diaphragm is mounted on a diaphragm support 34. This diaphragm support from the connection between the high-voltage section, i.e., the accelerator tube 10 and the bending magnet. A feedthrough 35 for the diaphragm support may also serve as a pump connection.

FIGS. 2 and 3 give a more detailed view of the l double-focussing bending magnet. In the bending magnet 16 an electron beam 8 which enters the stabilising bending device through the entrance diaphragm 17 is bent by a homogeneous magnetic field denoted by an arrow H at a radius R0 between, for example, 25 and cm. A collector element in the form of a foil 18 scatters a fraction of the electrons from the electron beam 8 and a detector 19 collects part of the dispersed electrons. Velocity variations in the electron beam result in a displacement of the electron beam at the collector element in a direction denoted by an arrow p. By means of a detector signal which varies as a function of this displacement the velocity of the electron beam is stabilized, and the electron beam is directed along the optical axis 15 of the electron-beam apparatus at the area of the collector element 18.

FIG. 3 is a diagrammatic cross-sectional view of a bending magnet taken at right angles to the beam path. On two sides of the optical axis 15 flat pole shoes 37 and 38 are provided. Imaginary flat planes 39 and 40 containing these pole shoes slope towards each other in the direction of the inner curve of the bending magnet so that they have a common point 41, which is situated at a distance of 2R0 from the optical axis. Two magnetic coils 42 and 43 then generate a double-focussing magnetic field of the type described in German Patent Specification No. 91 1,878 in a space 44 about the optical axis 15. In the embodiment shown, the pole shoes 37 and 38 and the electromagnetic coils 42 and 43 are mounted in a block 47 of electrically conducting material, for example aluminum, which consists of two symmetrical portions 45 and 46. Around the block 47 ferromagnetic hoods 48 and 49 are provided. The vacuum in the bending magnet required for the electron beam may be restricted to the inside of a bent tube 50 which coaxially surrounds the optical axis 15 in the space 44. In order to avoid stray fields at the ends of the bending magnet, a U-shape which bends away from the optical axis may be imparted to the coil windings at that area. The electromagnetic coils 42 and 43 may be replaced by permanent magnets.

FIG. 4 is a block diagram of the stabilizing circuit. The signal which is formed in the beam path between the entrance diaphragm 17 and the collector element 18 and which is received by the detector 19 is applied to an electrical circuit 51. A signal amplified by the circuit 51, which may be converted by comparison with adjustable fixed reference signals to a signal which is independent of the beam diameter and the beam current at the area of the collecting element is used, for example, for controlling the potential of the electron source 7, at a fixed potential of the last electrode 52 of the accelerator tube. Since a potential difference of the order of magnitude of l0 volts exists between the detector 19 and the electron source 7, a device 53 is added to bridge this difference. This device comprises an opto-electronic converter. The electromagnetic radiation signal supplied by this converter is applied to a photoamplifier tube 54 which is at a potential level which differs from the detector potential. The device 53 may also comprise other means for bridging the potential difference, such as a series connection of'transformers which are arranged in a mutually insulated manner. ln that case the photoamplifier tube may be dispensed with. A signal supplied by the amplifier tube 54 is applied to a variable auxiliary voltage source 55 by means of which the potential of the electron source 7 is controlled. In the embodiment shown in FIG. 4, the variable auxiliary voltage source is connected between the output electrode 5 of the high-voltage generator and a first electrode 56 of the accelerator tube, a constant second auxiliary voltage source 57 being situated between the electron source 7 and the first electrode of the accelerator tube. FIG. 4 further shows a current control device 58 for the electron gun 7, an enclosure of the portion of the apparatus in which a potential prevails which differs by approximately 10 V from earth potential. and which is indicated by a broken line 59, a sequence of voltage dividers 60 of the Van de Graaff generator, the ring electrodes 11 of the accelerator tube, and the image producing device in which the stabilized electron beam 8 is used.

What is claimed is:

1. An electron beam apparatus comprising a highvoltage generator having an end-electrode, an electron source for generating an electron beam, an accelerator tube for accelerating the electron beam, a beam stabilizing system and an electron-optical imaging system, said beam stabilizing system comprising a doublefocussing magnetic beam bending device having a magnetic field which is directed at right angles to the electron beam for bending the electron beam which is accelerated by means of the high-voltage generator, an entrance diaphragm which defines the beam diameter preceding the bending device, and a collecting element following the bending device approximately at the location of an image of the entrance diaphragm produced by the bending device.

2. An electron beam apparatus as claimed in claim 1, wherein the collecting element is arranged to intercept only a comparatively small part of the beam and is provided with a bounding edge which intersects the periphery of the beam.

3. An electron beam apparatus as claimed in claim 2, wherein the collecting element is a foil which scatters the intercepted electrons.

4. An electron beam apparatus as claimed in claim 2, wherein the collecting element consists of two electrically discrete parts which each have a bounding edge intersecting the beam and intercept substantially opposed periphery parts of the beam.

5. An electron beam apparatus as claimed in claim 1 wherein the largest diameter of the entrance diaphragm is at least approximately 5pm smaller than the central part of the electron beam arriving there and having a substantially constant current density.

6. An electron beam apparatus as claimed in claim 1 wherein tthe entrance diaphragm is circular and has a diameter of approximately 50pm.

7. An electron beam apparatus as claimed in claim 1 including electron-optical means between the electron source and the entrance diaphragm for directing to the idaphragm and producing a first image of the most constricted section of the electron beam which occurs near the electron source.

8. An electron beam apparatus as claimed in claim 1 wherein adjacent the collector element, which is in the form of a foil, there is disposed a detector having a sig nal output for detecting the electrons of the beam which have been scattered by the collecting element.

9. An electron beam apparatus as claimed in claim 8, wherein the detector is adjustable so that the angle between the axis of the beam of image-producing elec trons at the area of the collecting element and a line joining the foil and the detector is adjustable between approximately 20 and 10. An electron beam apparatus as claimed in claim 8 wherein the detector is a semiconductor diode which is connected in the reverse direction.

11. An electron beam apparatus as claimed in claim 10, wherein the semiconductor diode by application of a suitable reverse voltage, produces a sufficiently thick barrier layer in order to impart a self-amplification of at least 10 to the electrons which are collected by the diode and have an energy of between approximately 3 X 10 and 10 eV. I

12. An electron beam apparatus as claimed in claim 1 wherein the collecting element is a detector having an electrical signal output for the electrons intercepted from the beam.

113. An electron apparatus as claimed in claim 8 including means for controlling the potential of the electron source in accordance with the detector signal, at a constant potential of the last electrode of the accelerator tube for the electron beam.

14. An electron beam apparatus as claimed in claim 13, including a circuit between the electron source and the output electrode of the high-voltage generator comprising a variable auxiliary voltage source which is controlled in accordance with the detector signal.

15. An electron beam apparatus as claimed in claim 14, including a variable auxiliary voltage source connected between the output electrode of the highvoltage generator and a first electrode of the accelerator tube and a constant second auxiliary voltage source connected between the electron source and said first electrode.

16. An electron beam apparatus as claimed in claim 1, wherein the radius of the bend in the beam path has a length between approximately 25 cm and approximately cm.

17. An electron beam apparatus as claimed in claim 1, wherein the double focussing magnetic bending device is formed by a bending magnet.

18. An electron beam apparatus as claimed in claim 17, wherein the bending magnet comprises pole shoes having end surfaces which face the beam path and are situated on both sides of the beam path in planes which have a common point on the bisector of the inner bend in the beam path at a distance from the center of the beam path which is equal to twice the radius of the mean beam path.

19. An electron beam apparatus as claimed in claim 18, wherein the bending magnet includes a block of a non-ferromagnetic material which laterally embraces the beam path, the pole shoes engaging the two sides of this block which are parallel with the planes having a common point, while two wire windings extend parallel to the two sides which follow the bend of the beam path, ferromagnetic material which forms magnetic cirlowing a U-shape which differs from the beam path near the ends of the bend in the beam path.

21. An electron beam apparatus as claimed in claim 17 wherein a vacuum space enveloping the beam path is formed by a bent tube of non-ferromagnetic material which laterally embraces the beam path. 

1. An electron beam apparatus comprising a high-voltage generator having an end-electrode, an electron source for generating an electron beam, an accelerator tube for accelerating the electron beam, a beam stabilizing system and an electronoptical imaging system, said beam stabilizing system comprising a double-focussing magnetic beam bending device having a magnetic field which is directed at right angles to the electron beam for bending the electron beam which is accelerated by means of the high-voltage generator, an entrance diaphragm which defines the beam diameter preceding the bending device, and a collecting element following the bending device approximately at the location of an image of the entrance diaphragm produced by the bending device.
 2. An electron beam apparatus as claimed in claim 1, wherein the collecting element is arranged to intercept only a comparatively small part of the beam and is provided with a bounding edge which intersects the periphery of the beam.
 3. An electron beam apparatus as claimed in claim 2, wherein the collecting element is a foil which scatters the intercepted electrons.
 4. An electron beam apparatus as claimed in claim 2, wherein the collecting element consists of two electrically discrete parts which each have a bounding edge intersecting the beam and intercept substantially opposed periphery parts of the beam.
 5. An electron beam apparatus as claimed in claim 1 wherein the largest diameter of the entrance diaphragm is at least approximately 5 Mu m smaller than the central part of the electron beam arriving there and having a substantially constant current density.
 6. An electron beam apparatus as claimed in claim 1 wherein the entrance diaphragm is circular and has a diameter of approximately 50 Mu m.
 7. An electron beam apparatus as claimed in claim 1 including electron-optical means between the electron source and the entrance diaphragm for directing to the idaphragm and producing a first image of the most constricted section of the electron beam which occurs near the electron source.
 8. An electron beam apparatus as claimed in claim 1 wherein adjacent the collector element, which is in the form of a foil, there is disposed a detector having a signal output for detecting the electrons of the beam which have been scattered by the collecting element.
 9. An electron beam apparatus as claimed in claim 8, wherein the detector is adjustable so that the angle between the axis of the beam of image-producing electrons at the area of the collecting element and a line joining the foil and the detector is adjustable between approximately 20* and 80*.
 10. An electron beam apparatus as claimed in claim 8 wherein the detector is a semiconductor diode which is connected in the reverse direction.
 11. An electron beam apparatus as claimed in claim 10, wherein the semiconductor diode by application of a suitable reverse voltage, produces a sufficiently thick barrier layer in order to impart a self-amplification of at least 105 to the electrons which are collected by the diode and have an energy of between approximately 3 X 105 And 106 eV.
 12. An electron beam apparatus as claimed in claim 1 wherein the collecting element is a detector having an electrical signal output for the electrons intercepted from the beam.
 13. An electron apparatus as claimed in claim 8 including means for controlling the potential of the electron source in accordance with the detector signal, at a constant potential of the last electrode of the accelerator tube for the electron beam.
 14. An electron beam apparatus as claimed in claim 13, including a circuit between the electron source and the output electrode of the high-voltage generator comprising a variable auxiliary voltage source which is controlled in accordance with the detector signal.
 15. An electron beam apparatus as claimed in claim 14, including a variable auxiliary voltage source connected between the output electrode of the high-voltage generator and a first electrode of the accelerator tube and a constant second auxiliary voltage source connected between the electron source and said first electrode.
 16. An electron beam apparatus as claimed in claim 1, wherein the radius of the bend in the beam path has a length between approximately 25 cm and approximately 100 cm.
 17. An electron beam apparatus as claimed in claim 1, wherein the double focussing magnetic bending device is formed by a 180* bending magnet.
 18. An electron beam apparatus as claimed in claim 17, wherein the bending magnet comprises pole shoes having end surfaces which face the beam path and are situated on both sides of the beam path in planes which have a common point on the bisector of the inner bend in the beam path at a distance from the center of the beam path which is equal to twice the radius of the mean beam path.
 19. An electron beam apparatus as claimed in claim 18, wherein the bending magnet includes a block of a non-ferromagnetic material which laterally embraces the beam path, the pole shoes engaging the two sides of this block which are parallel with the planes having a common point, while two wire windings extend parallel to the two sides which follow the bend of the beam path, ferromagnetic material which forms magnetic circuits being disposed against these latter sides at the outer side thereof when viewed from the beam path.
 20. An electron beam apparatus as claimed in claim 19, wherein the wire windings on both sides of the beam path form part of two wire coils which are arranged substantially symmetrically with respect to a plane containing the center of the beam path in the bending magnet, the return windings of the coils following a U-shape which differs from the beam path near the ends of the bend in the beam path.
 21. An electron beam apparatus as claimed in claim 17 wherein a vacuum space enveloping the beam path is formed by a bent tube of non-ferromagnetic material which laterally embraces the beam path. 